Measurement and identification of nonlinear systems consisting oflinear dynamic blocks and one static nonlinearity

This paper considers the measurement and the identification of nonlinear time-invariant single-input/single-output (SISO) systems, consisting of a multivariable linear dynamic system and one static nonlinear SISO system. This includes Wiener-Hammerstein systems in a linear feedback loop. The nonparametric identification of the frequency response functions of the linear parts are obtained without measuring the signals over the static nonlinearity. Measurements on an electronic circuit demonstrate the usability of this identification scheme     

Robust broadband periodic excitation design

This paper considers a rather practical problem arising when inexperienced users misuse the (otherwise well-designed) periodic broadband excitation signals during the measurement or signal processing phase of an identification process. Using a fractional period of the excitation signal instead of full periods may affect not only the precision because of the well-known leakage effect, but may cause a serious loss of information on the measured system as well. The power of the excitation signal in certain frequency bands may be much lower (30-40 db) than it would be expected, and thus the measurement in a noisy environment may give poor result. A solution is presented here to make periodic broadband (multisine) excitation signals more robust against such misuse. The suggested solution is analyzed, and the theoretical results are verified by practical examples     

Parametric identification of two-port models in the frequencydomain

The authors treat the problem of parametric estimation of linear time-invariant dynamic two-port models (e.g. the short-circuit admittance matrix) from experimental data. A multivariate frequency-domain Gaussian maximum likelihood estimator is proposed to estimate the unknown coefficients occurring in the rational two-port model. It takes the perturbing noise of all the measured voltages and currents into account. The covariance matrix of the noise is assumed to be known, e.g. from measurements. The estimates and their covariance matrix are obtained as the result of an optimization procedure. The value of the minimized loss function and the covariance matrix of the estimates can be used to determine the model structure. The ability of the estimator to handle real measurement problems is demonstrated by means of experimental results. Using the estimated two-part parameters of an unloaded band-pass filter, it was possible to predict the transfer function of the loaded filter within an error of ±0.01 dB on the magnitude and ±0.1° on the phase     

The geometric mean of power (amplitude) spectra has a much smallerbias than the classical arithmetic (RMS) averaging

The statistical properties of the geometric mean of power (amplitude) spectra resulting from a discrete Fourier transform (DFT) are compared with those of arithmetic (RMS) averaging. The statistical properties are verified by means of frequency-domain and time-domain simulations. It is shown that the asymptotic bias of the geometric mean is a function of the fourth-order moments of the measurement noise     

A critical note on histogram testing of data acquisition channels

Histogram testing is used to measure the transfer characteristic of a digitizer in order to quantify and to analyze its nonlinear behavior. In this paper it is shown that this method is very insensitive to the out-of-phase components of the digitizers' response to a sine wave excitation. This can lead to an underestimate of the nonlinear behavior of the digitizer     

Measuring a linear approximation to weakly nonlinear MIMO systems

The paper addresses the problem of preserving the same LTI approximation of a nonlinear MIMO (multiple-input multiple-output) system. It is shown that when a nonlinear MIMO system is modeled by a multidimensional Volterra series, periodic noise and random multisines are equivalent excitations to the classical Gaussian noise, in a sense that they yield in the limit, as the number of the harmonics M→∞, the same linear approximation to the nonlinear MIMO system. This result extends previous results derived for nonlinear SISO (single-input single-output) systems. Based upon the analysis of the variability of the measured FRF (frequency response function) due to the presence of the nonlinearities and the randomness of the excitations, a new class of equivalent input signals is proposed, allowing for a lower variance of the nonlinear FRF measurements, while the same linear approximation is retrieved.     

Friction and Wear Mechanisms of Sintered and Thermoplastic Polyimides under Adhesive Sliding

Polyimides function under high‐temperature sliding. The available literature explains transitions in friction and wear mainly by mechanical effects, such as influences of normal load, sliding velocity and humidity on polymer transfer to steel counterfaces. Theoretical models are evaluated for sintered and thermoplastic polyimides. Tribologists have been interested in tribochemical and tribophysical reactions in the sliding interface for about 25 years. Reactions such as hydrolysis, imidisation and/or degradation occur as a function of sliding temperature and are reviewed in this paper. An overview of polyimide synthesis and degradation is presented, while new insights in sliding mechanisms are obtained from a detailed study of Raman spectroscopy on worn polymer surfaces.

     

Identification of continuous-time systems with missing data

This paper treats the identification of continuous-time systems with missing data in the input and output signals. A frequency-domain solution based on an extended transfer function model is given. The basic idea of the approach is to treat the missing data as unknown parameters in the identification problem. No particular pattern for the missing data is assumed. The method is illustrated on simulation and real measurement examples     

The friction and wear of different polymers under high‐load conditions

Engineering polymers are increasingly being used as self‐lubricating materials in demanding high‐load applications. In this study, a comparison has been made of the tribological behaviour under high‐load conditions of two grades of polyamide and polyethylene terephthalate as well as polyacetal. Large‐scale testing is used to reduce edge effects. It has been found that the two grades of polyamide and pure polyethylene terephthalate are prone to stick—slip, while internally lubricated polyethylene terephthalate and polyacetal show low friction and wear and no stick—slip.